Computer processors are increasingly becoming more complex and capable of performing many tasks simultaneously. At the same time, computer processors are now operating at higher speeds and able to input and output greater amounts of data. At the same time, the voltage level of which computer processors and other devices has decreased. For example, many computer processors today use a full 32 bit bus for data input and output while operating at very high speeds. The voltage requirement is also reduced from 5 volts to lower voltages, such as 3.3 volts, are lower. There is therefore a need for memory devices which are capable of transferring 32 bits of data simultaneously while at the same time operating at very high speeds and at lower voltage levels. The high speed of operation is required not only for reading data from the memory devices but also for writing data to the memory devices under control of the microprocessor.
At low supply voltages, for example 3.3 V and lower, correctly writing data to a memory cell is subject to more sensitivity than at higher supply voltages, such as 5 V. In addition, writing to memory cells at a lower voltage may require more time than writing at a higher voltage and may also require more time than reading data from the same memory cell.
At lower voltages, the body effect of a transistor has a greater affect on the operation of the transistor itself. In addition, the threshold voltage becomes a higher percentage of the total supply voltage when operating on a lower supply voltage. Because of these, and many other factors, the voltage available for storing data using a lower supply voltage is much less than is available when operating at the higher supply voltages.
A further problem with operating at lower voltages is that the speed at which data can pass through a transistor while the transistor is being turned ON is reduced because both the gate voltage and the data voltage are reduced from that which would be available at a higher voltage.
Even minor variations from one memory device to another may affect the time required to store data in seemingly identical memory cells. For example, even using the same manufacturing process, the bit line resistance may vary from one lot to another. Further, the contact resistance, whether buried contact or via, may vary slightly from one lot to another. Variations in the properties of the active areas, such as the bird's beak, the threshold voltage and the body effect may also cause variations in the time required to write data to a memory cell. At lower voltages, even very slight changes in any one of these parameters will affect the speed at which data can be correctly written to the memory cell.
According to the prior art, the time permitted to perform a write to a memory cell was established according to the product specifications for a given product design. Enough time was provided to ensure that data was written to the properly manufactured memory cell. If data cannot be correctly written to the memory cell in the time established by the specifications, the entire memory chip is deemed defective and cannot be sold. By establishing the product specifications at a long write time, the number of defective chips is reduced. However, this results in a slower speed of the memory devices. As faster and faster computer processors become available, the speed at which memory can be accessed is an important parameter for the commercial success of the memory on the market. It is therefore desirable to provide a memory in which data can be written to as quickly as possible while at the same time providing a memory which meets product specifications so that the chips do not need to be discarded.